1. Technical Field
The present invention relates to a Bragg grating sensor system in which resonance characteristics of a grating reflector are tailored to provide each sensor element in a series of elements with a unique spectral identity, or signature. Unlike normal grating sensors, where the gratings need to be separated in wavelength, this feature allows several gratings to occupy the same wavelength space. The overlapping spectral features of each grating are detected via the use of a spectral, matched filter correlation technique.
2. Description of Related Art
FIG. 1 shows a sensor system that is known in the art and includes fiber Bragg grating based sensors that provide a wavelength encoded mode of operation. To multiplex several sensors along a fiber, each grating is typically assigned a certain wavelength range over which it is to operate. This method of wavelength division multiplexing (WDM) limits the number of sensors which can be multiplexed, particularly if the gratings are subjected to large strains or temperature changes which give rise to large wavelength shifts. For example, a fiber strain of about +/xe2x88x921% requires an operational range of greater than +/xe2x88x9210 nanometers at 1.3 micrometers. For a source of 40 nanometer bandwidth, only 2 sensors could be accommodated under the source profile with this strain monitoring range sequence. In effect, in FIG. 1 the basic mode of operation of a wavelength division multiplexing (WDM) system (prior art) uses narrow spectral responses from each grating to make the measurement and provide discrimination between sensors.
In using Bragg gratings as sensors, it is often advantageous to maximize the responsivity, or scale factor. For example, in the transduction of pressure to strain, the larger the pressure-to-strain conversion factor (within constraints set by possible breakage of the fiber), the lower the minimum detectable pressure change will be for a given wavelength resolution capability. Unfortunately, when multiplexing grating sensors, the limited wavelength range that can be assigned to each sensor limits the responsivity that can be used.
The present invention provides a new and unique sensor system for sensing a parameter, comprising an optical source, coupler and signal processor system in combination with multiple structured fiber Bragg gratings.
The optical source, coupler and signal processor system provides an optical source signal to the multiple structured fiber Bragg gratings. The optical source, coupler and signal processor system also responds to multiple structured fiber Bragg grating signals, for providing an optical source signal, and also for providing an optical source, coupler and signal processor system signal containing information about a sensed parameter.
The multiple structured fiber Bragg gratings respond to the optical source signal, and further respond to the sensed parameter, for providing the multiple structured fiber Bragg grating signals containing information about a complex superposition of spectral responses or codes related to the sensed parameter.
Each of the multiple structured fiber Bragg gratings has a different spacing of one or more spectral components that are used to discriminate between the multiple structured fiber Bragg gratings.
Each of the multiple structured fiber Bragg gratings includes a respective broadband spectral response or code related to the sensed parameter. For example, each of the multiple structured fiber Bragg gratings may have separate gratings with either a unique broadband, multi-component, spectral response or spectral code. Each of the multiple structured fiber Bragg gratings may have a respective noise code. The respective noise code may include either maximal sequence codes or Gold codes.
The optical source, coupler and signal processing system may include either a broadband source or a scanning laser, or a broadband source with a scanning filter.
The optical source, coupler and signal processing system may also include either a scanning filter or wavelength resolving instrument to detect a net spectral response from the multi-structured fiber Bragg grating combination, or a simple detector, depending on the source as described above.
In contrast to the prior grating based sensor system, in the present invention each grating in the arrangement has a broadband response, typically giving a multi-component response. This type of response can be produced by writing a grating through a suitable amplitude mask. This type of grating is characterized by a central peak and a series of xe2x80x9csidebandxe2x80x9d peaks. These sidebands result due to the amplitude superstructure modulation of the grating. The spacing between the grating spectral components depends on the period of the superstructure amplitude mask period. The grating produced has a nominal center wavelength, or centroid wavelength which shifts with grating temperature or strain as with a xe2x80x9cnormalxe2x80x9d narrowband grating.
In addition, the present invention also provides an approach to the multiplexing of gratings using a spectral coding approach which allows gratings to be used over the same wavelength range. The sensors are not wavelength division multiplexed (WDM), but spectral-code division multiplexed (SCDM).
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.